From the state of the art it is known that the wear characteristics of cutting inserts which are comprised for example of a hard metal, a cermet, a ceramic or steel, can be improved by a coating. Especially with hard metal cutting inserts for turning, milling, boring, thread cutting and reaming, there are known coatings of carbides, nitrides, carbonitrides, oxycarbonitrides, oxides and/or borides of at least one of the elements of Groups IVa to VIa of the Periodic System as are ceramic coatings. As examples mention may be made of titanium carbide, titanium nitride, and/or aluminum oxide. The thicknesses of such layers as a rule amounts to 5 to 15 .mu.m. Uncoated as well as coated cutting inserts are usually cooled by means of liquid cooling lubricants during the machining to reduce or carry away the heat resulting from chip formation. The use of liquid cooling lubricants can however counteract an effective corrosion protection as long as one cannot be certain that no reactions between the compounds contained in the cooling lubricant with the workpiece surface can result. Apart from this, however, liquid cooling lubricants are in part detrimental to health and must be disposed of in an expensive manner.
There have already been proposals to avoid liquid cooling lubricants and instead of them to provide conditions under which the tools can operate dry. For example, in U.S. Pat. No. 4,619,866, a process for producing a substrate body of a hard metal or a ceramic with an Al.sub.2 O.sub.3 coating is described in which the substrate body is subjected to a gas phase containing one or more metal halogenides at 7000 to 1200.degree. C. in a CVD process. To increase the layer growth of the Al.sub.2 O.sub.3, a substance selected from the group which consists of sulphur, selenium, tellurium, phosphorous, arsenic, antimony, bismuth or mixtures thereof is added to the gas phase in an amount of 0.03 to 1 volume %. For example, H.sub.2 S can be added for both an increase in the deposition speed and also to give rise to the formation of a aluminum oxide and .kappa. aluminum oxide. From a point of view of process technology, however the use of H.sub.2 S is detrimental both because of its extremely unpleasant odor and also because it contaminates the apparatus with H.sub.2 S decomposition products.
A process which corresponds thereto is described also in EP 0 523 021 A1 in which a substrate body is coated with at least one of the carbides, nitrides, carbonitrides or oxycarbonitrides of elements in Groups IVa to VIa of the Periodic System and to which in the reaction gas 0.4% H.sub.2 S, Inter alia, can be added.
To generate a crack-free surface coating, EP 0 659 903 A1 proposes to add to the gas atmosphere which aside from hydrogen contains CO.sub.2, CO and AlCl.sub.3, an amount of 0.2% SF.sub.6, in addition to 4% HCl, the balance hydrogen.
EP 0 534 905 B1 describes a process for producing a machining tool which is coated with at least one compound comprised of at least one element of the group O, S, Se, Te on the one hand, and at least one Group V element, Nb, Ta, Cr, Mo and W on the other hand. Molybdenum disulfide especially can serve as a wear-reducing lubricant layer. The disadvantage of a pure molybdenum disulfide layer is that the limited hardness of the layer causes it to be rapidly worn off.
From DE 202 898 there is known, for example, a sequence of a hard material layer of TiC and a soft MoS.sub.2 layer, by means of which the advantage of the increased hardness of TiC can be linked with an improved frictional coefficient of the outer layer.
Similar advantages can apply for slide bearings made from stainless steels. According to DE 24 15 255 A1, for example, an increase of adhesion of the bearing surface by sputtered MoS.sub.2 or WS.sub.2 can be obtained by providing a base layer of a thickness of 1 to 2 .mu.m and then a further polished layer which can be comprised of MoS2, WS.sub.2 or a synthetic resin like polytetrafluoroethylene. A disadvantage of such layers is that they require an extensive PVD technology.